Introduction: Self-assembled hydrogels through host-guest chemistry are an emerging class of injectable materials with shear-thinning and self-healing properties that are useful for therapeutic delivery[1]. Towards further advancing these systems, the transition from bulk isotropic hydrogels towards microstructured materials provides several advantages, including improved cell attachment/spreading and multiple compartments for controlled therapeutic delivery[2]-[4]. Here, we developed a colloidal self-healing hydrogel based on the host-guest mediated self-assembly of cyclodextrin (CD) modified hyaluronic acid (HA) microgels with adamantane (Ad) modified HA polymer chains (Figure 1).
Materials and Methods: HA (90 kDa) was methacrylated following previously published protocols[5]. Methacrylated HA (MeHA) was then modified with CD through the reaction of the MeHA TBA salt with aminated CD. Ad-HA was synthesized following previously published protocols[1].
CD-MeHA microgels were synthesized using an emulsion technique. Briefly, CD-MeHA was dissolved in a solution containing a radical initiator, and emulsified in a cyclohexane continuous phase containing Span80 and Tween80 surfactants. Microgels were stabilized via UV initiated crosslinking of methacrylate groups. Finally, microgels were precipitated from the emulsion in acetone, and washed several times with acetone and water before lyophilization.
Rheology was performed using a stress-controlled rheometer to analyze oscillatory time sweeps, frequency sweeps, and high strain recovery profiles. Four groups were analyzed: microgels alone, Ad-HA alone, microgels + HA, and microgels + Ad-HA. Groups were formulated in dilute conditions, then lyophilized and reconstituted at higher concentrations to ensure thorough mixing of microgels and polymers.
Results and Discussion: Image analysis of fluorescently labeled microgels showed stable microgels with an average diameter of 4.25 ± 0.4 μm (Figure 1). Mechanical testing indicated that microgels + Ad-HA formed a network, with statistically significant higher G’ and G” over other groups at a 10Hz frequency (Figure 2A). Interestingly, microgels alone, microgels + HA, and microgels + Ad-HA behaved similarly over low frequencies, with microgels + Ad-HA maintaining significantly higher mechanics over higher frequency ranges (Figure 2D). Finally, colloidal hydrogels showed disassembly when subjected to high strains and rapid reassembly when the high strain was removed, indicating network self-healing (Figure 2C). With these properties the assemblies could be injected through a 28G needle (Figure 2D). Thus, CD-MeHA microgels successfully formed injectable dynamic networks with hierarchical structure through non-covalent interactions. An important aspect of the material design is the unique modularity, which is allowing us to pursue multi-compartmental delivery applications where individual particles can be tailored with specific therapeutics and design (e.g., mechanics). This feature is not possible with traditional bulk hydrogel design.
Conclusion: Here, we show the development of an injectable, microstructured material based on the host-guest mediated self-assembly of hydrogel particles. The dynamic nature of this material provides an interesting platform to investigate further as an injectable for therapeutic delivery and to control cell interactions.
References:
[1] Rodell, C. B., Kaminski, A. L., & Burdick, J. A. (2013). Rational Design of Network Properties in Guest–Host Assembled and Shear-Thinning Hyaluronic Acid Hydrogels. Biomacromolecules, 14(11), 4125-4134.
[2] Griffin, D. R., Weaver, W. M., Scumpia, P. O., Di Carlo, D., & Segura, T. (2015). Accelerated wound healing by injectable microporous gel scaffolds assembled from annealed building blocks. Nature Materials, 14(7), 737-744
[3] Appel, E. A., Tibbitt, M. W., Webber, M. J., Mattix, B. A., Veiseh, O., & Langer, R. (2015). Self-assembled hydrogels utilizing polymer–nanoparticle interactions.Nature Communications, 6.
[4] Wang, H., Hansen, M. B., Löwik, D. W., van Hest, J., Li, Y., Jansen, J. A., & Leeuwenburgh, S. C. (2011). Oppositely charged gelatin nanospheres as building blocks for injectable and biodegradable gels. Advanced Materials,23(12), H119-H124
[5] Khetan, S., Guvendiren, M., Legant, W. R., Cohen, D. M., Chen, C. S., & Burdick, J. A. (2013). Degradation-mediated cellular traction directs stem cell fate in covalently crosslinked three-dimensional hydrogels. Nature Materials,12(5), 458-465